Microwave power transistors are used in many applications, such as phased array radars and microwave communication systems. It is therefore desirable to increase the power output and frequency range of such devices without unduly increasing cost. The parasitic inductance of the leads to the transistor chip within the package and stray capacitance among the leads and to ground has been a limiting factor in the frequency response of microwave power transistors. In the past attempts to reduce lead length and thereby lower the inductances and capacitances due to the leads, decreased the maximum size of the transistor chip and the maximum available power output of the device was also correspondingly decreased.
In most microwave power transistors, the transistor chip is supported upon a ceramic substrate such as alumina (Al.sub.2 O.sub.3) or beryllia (BeO). these ceramic materials are chosen because of their relatively high thermal conductivity and relatively low electrical conductivity. The substrate, which is carefully cut and fit and underlies the entire frame of the transistor package, is mounted on the underlying relatively large thermally and electrically conducting base. In such devices, relatively large amounts of BeO or Al.sub.2 O.sub.3 were commonly used. Since alumina and beryllia are relatively expensive materials, the resultant device is expensive to fabricate. Achieving a ground connection between the transistor chip around or through the ceramic substrate is also a frequent problem. One method for connecting a ground terminal to the transistor chip is to drill precisely positioned holes through the ceramic substrate, fill the holes with a conducting material such as gold, and then connect the filled holes via a gold metallization pattern on the top of the substrate to the transistor chip. Such approaches commonly involve precise metallization patterns greatly increasing the fabrication costs of the device. On the other hand, if the entire ceramic substrate is metallized and ground connection made through the metallization layer, the path length and, hence, parasitic capacitance and inductance is correspondingly increased for then, the connection had to be made around the edges of the relatively large ceramic substrate. Another problem in present designs is caused by the criticality of the physical dimensions of the ceramic substrate which have to be precisely fitted into the surrounding structure. This also increases the fabrication costs. Impedance matching networks are needed before and after each transistor in a microwave transistor amplifier since, for the most part, the impedance of each transistor varied from that of the circuit in which is was being used. This lack of impedance matching increases the cost, size and complexity of the amplifier.